Haze-free BST films

ABSTRACT

Described herein is a method for producing a haze-free (Ba, Sr)TiO 3  (BST) film, and devices incorporating the same. In one embodiment, the BST film is made haze-free by depositing the film with a substantially uniform desired crystal orientation, for example, (100), preferably by forming the film by metal-organic chemical vapor deposition at a temperature greater than about 580° C. at a rate of less than about 80 Å/min, to result in a film having about 50 to 53.5 atomic percent titanium. In another embodiment, where the BST film serves as a capacitor for a DRAM memory cell, a desired {100} orientation is induced by depositing the bottom electrode over a nucleation layer of NiO, which gives the bottom electrode a preferential {100} orientation. BST is then grown over the {100} oriented bottom electrode also with a {100} orientation. A nucleation layer of materials such as Ti, Nb and Mn can also be provided over the bottom electrode and beneath the BST film to induce smooth, haze-free BST growth. Haze-free BST film can also be favored by forming the bottom electrode at high temperatures close to those used for BST deposition, and without a vacuum break between the bottom electrode and BST deposition.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 09/971,945,filed on Oct. 4, 2001, now U.S. Pat. No. 6,660,535, which is acontinuation of application Ser. No. 09/382,753, Aug. 25, 1999, now U.S.Pat. No. 6,319,764. This case is also related to application Ser. No.09/971,955, which is a divisional of application Ser. No. 09/382,753.The above related applications are incorporated herein by reference andmade part of the present Application.

REFERENCE TO GOVERNMENT CONTRACT

This invention was made with United States Government support underContract No. DABT63-97-C-0001, awarded by the Advanced Research ProjectsAgency (ARPA). The United States Government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to (Ba,Sr)TiO3 (BST) thin films, and moreparticularly to a method for creating a haze-free BST thin film with ahigh dielectric constant.

2. Description of the Related Art

(Ba,Sr)TiO₃ (BST) films are commonly used as dielectric materials forcapacitors, gate dielectrics and high frequency electronic circuits.More particularly, BST films have found application as capacitors indynamic random access memory (DRAM) cells. A typical DRAM cell comprisesa charge storage capacitor (or cell capacitor) coupled to an accessdevice such as a metal-oxide semiconductor field effect transistor(MOSFET). The MOSFET functions to apply or remove charge on thecapacitor, thus affecting a logical state defined by the stored charge.The amount of charge stored on the capacitor is determined by thecapacitance, C=∈∈₀A/d, where ∈ is the dielectric constant of thecapacitor dielectric, ∈₀ is the vacuum permittivity, A is the electrode(or storage node) area, and d is the interelectrode spacing. Theconditions of DRAM operation such as operating voltage, leakage rate andrefresh rate, will in general mandate that a certain minimum charge bestored by the capacitor.

BST is desirable for such applications because of its high dielectricconstant, low DC leakage, low dispersion up to high frequencies andstable operation at high temperatures. The high dielectric constant ofBST thereby gives the material the ability to yield high capacitancewhen placed between a pair of electrodes. BST films grown forapplications such as DRAM capacitors are typically made usingmetal-organic chemical vapor deposition (MOCVD) or sputtering. However,MOCVD growth of such films typically leads to problems such as hazewhich reduces the dielectric constant of the material and increasesleakage currents. Specifically, haze is caused by the growth ofspatially correlated, non-textured BST, which in turn createsdiscernible optical scatter and a cloudy or hazy appearance in the film.For example, haze may be created when a film desired to be grown in a(100) orientation has orientations other than (100), such as (110) or(111), thereby disrupting the texture of the film. When BST is used in acapacitor structure, haze causes its capacitance to decrease as much as50% and leakage currents to increase by a factor of 10 to 1000 withrespect to smooth films.

Furthermore, electrodes and other materials on which BST films aredeposited often suffer from process-induced defects such as hillockformation which may severely limit performance. Hillocks are smallnodules which form when the electrode or other material is deposited orsubjected to post-deposition processing. For example, hillocks canresult from excessive compressive stress induced by the difference inthermal expansion coefficient between the BST film and the underlyingelectrode material during post-deposition heating steps. Such thermalprocessing is typical in the course of semiconductor fabrication.Hillock formation may create troughs, breaks, voids and spikes along theelectrode surface, thereby leading to uneven BST growth and stress inthe BST film.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to produce a haze-freeBST thin film with a high dielectric constant and low leakage currents.It is also an object of this invention to produce a BST thin film thathas low stress. It is further an object of this invention to produce anelectrode or other base material onto which a BST film is deposited thatis smooth and hillock-free to improve the properties of the subsequentlydeposited BST film.

These objects are achieved generally through the control of one or moreprocessing conditions in the fabrication of the BST film. Brieflystated, haze can be reduced by increasing the BST depositiontemperature, decreasing the deposition rate and increasing the atomicpercent of titanium in the BST film. These conditions favor theformation of a highly textured film, i.e., a film with a substantiallyuniform desired crystal orientation. Furthermore, use of highly texturedsubstrates, bottom electrodes or nucleation layers also favors haze-freeand low stress BST films. Moreover, the above-stated objects areachieved by forming a substrate layer such as a bottom electrode and theBST film in a clustered tool.

In one aspect of the present invention, a method of forming a haze-freeBST film over a substrate assembly is provided. The method comprisessupplying BST sources into a chamber, and inducing textured growth ofthe BST film over the substrate assembly in a substantially uniformdesired crystal orientation. In one preferred embodiment, the BST filmis deposited at a rate of less than about 80 Å/min at a chambertemperature above about 580° C. The BST film is preferably grown usingmetal-organic chemical vapor deposition (MOCVD), and results in a filmhaving a concentration of about 50 to 53.5 atomic percent titanium.

In another aspect of the present invention, a substantially haze-freeBST thin film is provided. The BST thin film has a textured structurewith a substantially uniform crystal orientation.

In another aspect of the present invention, the method of forming thesubstantially haze-free BST film first comprises forming a nucleationlayer over a substrate assembly. Then, the BST film is formed over thenucleation layer, the BST film being formed having a substantiallyuniform crystal orientation. In one preferred embodiment, the nucleationlayer is NiO, and an orientation layer such as platinum is formed overthe nucleation layer before forming the BST film. The orientation layerpreferably has a desired crystal orientation to induce the sameorientation in the subsequently formed BST film. In another preferredembodiment, the nucleation layer is made of Ti, Nb or Mn to compensatefor defects in the subsequently formed BST film.

In another aspect of the present invention, a thin film structure isprovided comprising a nucleation layer and a BST film over thenucleation layer having a substantially uniform crystal orientation. Inone embodiment, an orientation layer is preferably provided over thenucleation layer underneath the BST film. In another embodiment, the BSTfilm is directly on top of the nucleation layer.

In another aspect of the present invention, a method of forming a BSTcapacitor structure is provided. A first electrode material is formedover a substrate assembly, followed by forming a BST film over the firstelectrode material. The BST film being formed has a substantiallyuniform crystal orientation. A second electrode material is then formedover the BST film. The first electrode material is preferably formed ina vacuum at a temperature between about 500 and 550° C., while the BSTfilm is preferably formed at a temperature greater than about 580° C.The BST film is preferably deposited in a vacuum chamber, with the firstelectrode material and the BST film formed without a vacuum break inbetween.

In another aspect of the present invention, a capacitor structure isprovided comprising a base layer, a bottom electrode formed over thebase layer, a BST film formed over the bottom electrode, and a topelectrode formed over the BST film. The BST film has a substantiallyuniform orientation, and preferably comprises between about 50 and 53.5atomic percent titanium. Preferably, a nucleation layer of NiO isprovided between the base layer, which is preferably polysilicon, andthe bottom electrode, which is preferably platinum. A second oralternative nucleation layer may be provided between the bottomelectrode and the BST film, and more preferably comprises a materialsuch as Ti, Mn or Nb.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the crystal structure of BST.

FIG. 2 is a schematic cross-sectional view of a capacitor structure foruse in a DRAM memory cell incorporating a BST thin film.

FIG. 3 is a schematic view of a MOCVD processing apparatus used todeposit a BST thin film.

FIG. 4A is a schematic cross-sectional view of a (100)-oriented BST thinfilm deposited over a platinum bottom electrode.

FIG. 4B is a schematic cross-sectional view of a polycrystalline BSTthin film deposited over a platinum bottom electrode.

FIG. 5 is a schematic cross-sectional view depicting the deposition of aBST thin film over a platinum bottom electrode using a nucleation layerof NiO.

FIG. 6 is a schematic cross-sectional view depicting the deposition of aBST thin film over a platinum bottom electrode using a nucleation layerof Ti, Nb or Mn.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Briefly stated, the objects of the present invention are accomplished byproviding methods and apparatus that favor the growth of textured BSTfilms with a substantially uniform desired crystal orientation. Thepreferred embodiments of the present invention describe a BST thin filmformed as a capacitor in a DRAM memory cell. However, it will beappreciated that the teachings disclosed herein are applicable to anymethod or device where a haze-free BST film with high dielectricconstant is desired. As used herein, the term BST can refer not only to(Ba,Sr)TiO₃, but also to BaTiO₃, SrTiO₃ and any modifications to thesematerials through isovalent substitution or the use of donor andacceptor dopants. Furthermore, the methods and apparatus taught hereinare applicable to other materials similar to BST where it is desired toreduce haze and maintain a high dielectric constant.

FIG. 1 illustrates the BST crystal structure. As can be seen, BST has acubic crystal structure, more particularly, a perovskite crystalstructure with Ba²⁺ and Sr²⁺ at the corners, O²⁻ at the faces and Ti⁴⁺at the center. The (100) planes of the BST crystal structure are shadedin FIG. 1. As described below, the illustrated embodiments of thepresent invention provide textured growth of the deposited BST filmalong (100) planes. However, this invention is not to be limited toproducing a BST film with only a substantially uniform (100)orientation. It will therefore be appreciated that the methods andprocessing conditions described herein are also applicable to producinga BST film with a desired crystal orientation along other planes in the{100} family, as well as planes in the {110}, {111} and other familiesof planes found in the BST crystal structure.

FIG. 2 illustrates a schematic capacitor structure 10 for use in a DRAMmemory cell. At the bottom of the cell 10 is a base layer 12, preferablymade of a material such as polysilicon. A bottom electrode or storagenode 14 is formed, preferably by chemical vapor deposition, over thebase material 12. This bottom electrode is preferably platinum (Pt) orruthenium (Ru). Other electrodes, such as Ir, IrO_(x), RuO_(x) Pt—Rh, Moand Pd may also be used. A thin film 16 of BST is deposited over thebottom electrode 14. A top electrode or cell electrode 18 is preferablydeposited by chemical vapor deposition over the BST thin film 16, and ispreferably made of material similar to the bottom electrode, such as Ptor Ru. In the exemplary capacitor structure 10, the bottom electrode 14is preferably grown to a thickness of about 400 Å. The BST film 16 grownover the bottom electrode 14 preferably has a thickness of about 150 to300 Å. At the top of the structure 10, the top electrode 18 preferablyhas a thickness of about 300 Å.

Optionally, a barrier layer may be formed between the polysilicon baselayer 12 and the electrode 14 to prevent interdiffusion between thelayers and the formation of SiO₂ on top of the electrode surface.Appropriate barrier layers include TiN/Ti, TiAlN, TaSiN, WSiN, and maybe formed by chemical vapor deposition or physical vapor deposition, aswould be known to one skilled in the art.

The BST film 16 is formed over the bottom electrode 14 preferably usingmetal-organic chemical vapor deposition (MOCVD). A schematic MOCVDapparatus 20 is illustrated in FIG. 3, including a chamber 21 with asubstrate assembly 22 located therein. Suitable BST sources, including abarium source 24, a strontium source 26, and a titanium source 28, areprovided into the chamber, as would be known to one skilled in the art.For example, suitable barium and strontium sources include Ba(THD)₂ andSr(THD)₂, where THD denotes 2,2,6,6-tetramethyl-3,5-heptanedionate.Suitable Ti sources include bis(isopropoxy)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) titanium(Ti(O-i-Pr)₂(THD)₂). These precursors may be adducted with tetraglyme.Solvents used for these precursors may be butyl acetate ortetrahydrafuran, as would be known to one skilled in the art. A carriergas 33 such as Ar is preferably used to carry vapor from the sourcesinto the reaction chamber 21. Oxidizers 30 and 32 of O₂ and N₂O gas,respectively, are preferably used.

Deposition of the BST film 16 is preferably conducted in a chamber 21 ata pressure of about 100 mtorr to 10 torr. In one preferred embodiment,it has been found that chamber temperatures greater than about 580° C.are effective in reducing haze. For high temperature BST processes,deposition preferably occurs at a chamber temperature of between about600 and 680° C., and a substrate assembly temperature of about 500 to580° C. For low temperature BST processing, deposition preferably occursat a chamber temperature of between about 400 and 500° C., and asubstrate temperature of about 350 and 450° C. The BST film 16 ispreferably deposited at a rate of about 10 to 100 Å/min, more preferablyless than about 80 Å/min.

The resulting BST film 16 has a titanium concentration of between about50 and 53.5 atomic percent. It has also been found that when titaniumconcentrations of about 50-52 atomic percent yield haze, increasing thetitanium concentration to about 52-53 atomic percent can reduce haze.The ratio of Ba to Sr in the resulting film is preferably between about70/30 to 50/50.

FIG. 4A illustrates a BST film 16 formed over a platinum electrode 14.The conditions described above preferentially form the BST film 16 witha substantially uniform orientation. For example, substantially all ofthe BST growth may be with a (100) orientation. In other words, thedirection of growth of the BST film 16 as indicated by arrow 15 isnormal to (100) planes in the BST film. This (100) orientation leads toa more uniform grain structure and a smoother surface, thereby producinga textured structure with a less cloudy or hazy appearance. By contrast,FIG. 4B illustrates a BST film 16 which is not uniformly oriented. Thestructure shown in FIG. 4B is polycrystalline, with growth occurring inmore than one direction and thus deviating from the preferredorientation. For example, when (100) is the desired orientation, thestructure shown in FIG. 4B may be caused by deviating growth with (110)or (111) orientations. This causes a more disrupted grain structure andan uneven surface, thereby leading to haze. It will be appreciated thatalthough the preferred embodiments describe growth with a substantiallyuniform (100) orientation, uniform growth with other orientations arecontemplated as well.

It has been found that the substantially uniform orientation illustratedin FIG. 4A is induced by the high temperatures, preferably above about580° C., used for BST deposition, which favors equilibrium. Moreover,the slow deposition rate used, preferably less than about 80 Å/min asdescribed above, favors the formation of a more stable film, while thehigh Ti concentration in the BST produced, preferably between about 50and 53.5 atomic percent, also favors a more haze-free film.

As a modification to the embodiments described above, the use of one ormore nucleation layers either above or below the bottom electrode 14, orboth, favors a desired direction of crystal growth. For example, whenthe BST thin film 16 is grown over a bottom electrode 14 such as Pt, adesired {100} orientation of the BST film can be favored by using a thinNiO nucleation layer or similar material. Normal growth of Pt over abase material 12 such as silicon is along the {111} planes of Pt. Thisleads to the problem that BST films grown over a Pt electrode 14 may notbe {100} oriented, but rather {111} oriented.

As shown in FIG. 5, this problem is avoided by depositing a thin layer34 of NiO over the base material 12 to induce {100}-oriented Pt. NiO maybe grown by sputtering or metal-organic chemical vapor deposition. TheNiO nucleation layer 34 preferentially induces a highly textured {100}orientation in the Pt layer 14. The Pt layer in turn acts as anorientation layer that induces the same orientation in the subsequentlydeposited BST film 16. This is because both Pt and BST havesubstantially the same lattice constant of about 3.9 to 4 Å. Thus, the{100} orientation of the Pt layer 14 is preferentially transferred tothe BST film 16. It is also contemplated that other nucleation ororientation layers may be used to induce growth of subsequent layerswith other desired crystal orientations.

In another modification to the embodiments described above, haze canalso be prevented by providing a thin nucleation layer over the bottomelectrode 14. As shown in FIG. 6, a thin layer 36 of material such asTi, Nb or Mn is grown over the bottom electrode 14. The layer 36 ispreferably deposited using a physical vapor deposition technique such assputtering to a thickness of less than about 50 Å. The layer 36preferentially induces a more uniform, haze-free BST film 16 bycompensating for defects formed in the subsequently deposited BST film.More particularly, the nucleation layer 36 can act as either a donor oracceptor dopant to correct for defects in the BST film. The nucleationlayer 36 also impacts the nucleation kinetics of the BST film to enablea more uniform growth.

In another embodiment, the properties of the BST film formed accordingto the embodiments described above can be improved by forming a smootherbottom electrode layer 14, which thereby leads to an improved haze-freeBST film. In particular, by reducing the number of hillocks andincreasing the smoothness of the bottom electrode layer 14, thedeposited BST film will experience less stress and have a more uniformgrowth. Preferably, the smoothness of the bottom electrode 14 isaccomplished by depositing the electrode at a high temperature close tothat of the BST deposition. More particularly, it has been found thatfor high temperature BST processing, depositing the bottom electrode attemperatures as high as about 500 to 550° C. reduces the stress in thesubsequently deposited BST film.

The bottom electrode 14 is also preferably deposited using a clustereddeposition technique. By this technique, the bottom electrode 14 isdeposited under vacuum conditions, and then the BST film 16 is depositedthereover without a vacuum break. By eliminating the vacuum break andforming both the bottom electrode and the BST film in a clustered tool,it has been observed that the number of hillocks in the bottom electrode14 is reduced, while also reducing the stress in the layer 14.

It should be appreciated that the embodiments described above are purelyexemplary, and various modifications can be made as would be known toone of skill in the art. Accordingly, the scope of the present inventionshould be defined only by the claims that follow.

1. A method of forming a substantially haze-free BST film over asubstrate assembly, comprising: forming an electrode material over thesubstrate assembly; forming a nucleation layer over the electrodematerial, wherein the nucleation layer consists essentially of a metal;and forming a BST film over the nucleation layer.
 2. The method of claim1, wherein the nucleation layer consists essentially of a member of thegroup consisting of Ti, Nb, and Mn.
 3. The method of claim 1, whereinthe electrode material is Pt.
 4. The method of claim 1, wherein thenucleation layer is formed to have a thickness of less than about 50 Å.5. The method of claim 1, wherein the BST film is deposited at a rate ofbetween about 10 and about 100 Å/min.
 6. The method of claim 1, whereinthe BST film is deposited at a rate of about 80 Å/min.
 7. The method ofclaim 1, wherein the BST film is formed such that it contains betweenabout 50 and about 53.5 atomic percent Titanium.
 8. The method of claim7, wherein the BST film is formed such that it contains about 52 to 53atomic percent Titanium.
 9. A method of forming a substantiallyhaze-free BST film over a substrate assembly, comprising: forming afirst electrode material over the substrate assembly; forming anucleation layer over the first electrode material; forming a BST filmover the nucleation layer; forming a second electrode material over theBST film; wherein the nucleation layer consists essentially of a metalselected from the group consisting of: Ti, Nb, and Mn.
 10. The method ofclaim 9, wherein the nucleation layer is formed directly over the firstelectrode material.
 11. The method of claim 9, wherein the resulting BSTfilm comprises between about 50 and about 53.5 atomic percent titanium.12. The method of claim 9, wherein the resulting BST film comprisesbetween about 52 and about 53 atomic percent titanium.
 13. The method ofclaim 9, wherein the BST film is between about 150 and about 300 Åthick.
 14. The method of claim 9, wherein the BST film is deposited at arate of about 80 Å/min.
 15. The method of claim 9, further comprisingraising the temperature of the substrate assembly to about 500 to 550°C.